(1) Field of the Invention
The present invention relates to an optical filter for absorbing near infrared rays and more particularly to a high-purity green anthraquinone which has an absorption band which rapidly terminates in a limited wavelength range in the near infrared region above 700 nm.
(2) Description of the Prior Art
In recent years, various electric devices and measuring equipment have the inclination of diversification and many functions together with the development of electronic technique. In such circumstance, colorants which have an absorption band in the near infrared region are extended in use such as near infrared rays absorbing filters for sensors, sunglasses, protective glasses and optical recording media. Particularly, in case of optical filters for the sensors or the like consisted of this kind of colorant, there is an increasing demand which do not absorb near infrared rays or laser beams in a certain wavelength range but which absorb and cut out rays in another certain wavelength range.
In order to use colorants consisted in such optical filters, fairly high purity is required. However conventional near infrared rays absorbing colorants have low purity and inadequate absorption characteristics and thus can scarecely be used. Particularly in case of a 1,4,5,8-tetraanilidoanthraquinone, the allowable limit of its transmittance in the vicinity of 860 nm is at a level of about 95 to about 96% (a concentration of the dissolved colorant at the time of the transmittance measurement is 4 mg per 100 ml of chloroform). If its transmittance is below this level, the transmission of semiconductor laser beams is hindered noticeably, and therefore it is difficult to use such a colorant as a near infrared rays absorbing filter for measuring equipment and the like where particularly high accuracy is required.